At the present time, virtually all printed copy is produced through the use of three basic types of printing plates. One type is a relief plate which prints from a raised surface. Another type is an intaglio plate which prints from a depressed surface. The third type is the lithographic plate which prints from a substantially flat surface which is neither appreciably raised above nor appreciably depressed below the adjacent and surrounding non-printing areas. Printing is occasioned by an ink's respective affinity and/or aversion to areas of different chemical properties. Lithographic printing plates are commonly processed to have water-repellent (hydrophobic), oil-receptive (oleophilic) image areas and waterreceptive (hydrophilic) non-image areas.
Prior to processing for use, conventional lithographic plates will typically have a hydrophobic, photoreactive polymeric layer (i.e. photoresist) coated or otherwise deposited atop a hydrophilic substrate.
In preparing a conventional lithographic plate for use on a printing press, the plate is first exposed to actinic radiation. Specific chemical reactions are caused to occur in the plate's photoresist by exposure to actinic radiation. Such photoinduced chemical reactions may either reduce or enhance the solubility of the photoresist, depending on whether the resist is negative-working or positive- working. In negative-working plates, exposure to actinic radiation will generally cause a "hardening" of the photoresist. In positive-working plates, exposure to actinic radiation will generally cause a "softening" or "solubilization" of the photoresist.
After photoexposure, a wet development step is normally conducted. The objective of such wet development is to provide preferential solvation of the areas of the photoresist which have undergone photoinduced chemical change. Solvation under conventional development techniques will typically involve treating the exposed plate with organic solvents in a developing bath. For negative-working resists, the solvent will swell and dissolve the unexposed portions of the resist. The solvent should not swell the exposed portions or distortion of the developed image may result. For positive-working resists, the response of the unexposed and exposed coatings are reversed, but the same general principles apply.
As a result of the preferential solvation and washing away of portions of the photoresist, corresponding portions of the underlying hydrophilic substrate are uncovered. For negative-working plates, the aforementioned hydrophobic image areas correspond to the portions of the photoresist remaining after solvation and washing. The aforementioned hydrophilic non-image areas correspond to uncovered portions of the substrate. The image and non-image areas thus differentiated, the processed plate may then be mounted onto a printing press and run.
Encumbered by required wet development, the processing of conventional lithographic plates prior to their use on a printing press is time and labor consuming and involves the use of substantial quantities of organic chemicals. It will be appreciated that there is considerable attractiveness for innovations that would satisfactorily eliminate or reduce conventional lithography's long-felt dependency upon the conduct of bath development and thereby permit the use of lithographic plates on a printing press immediately after exposure without required intermediary processing.
In the past, dry-developable lithographic printing plates have been suggested which enable the wet processing steps of lithographic printing plates after exposure to be omitted and printing to be conducted by directly mounting the exposed plates on a printing press. Among printing plates that may be characterized as "on-press" developable (or related thereto) are: e.g., U.S. Pat. No. 4,273,851, issued to Muzyczko et al. on Jun. 16, 1981; U.S. Pat. No. 5,258,263, issued to Z. K. Cheema, A. C. Giudice, E. L. Langlais, and C. F. St. Jacques on Nov. 2, 1993; and U.S. Pat. No. 5,395,734, issued to Vogel et al. on Mar. 7, 1995.
Despite the methodologies and approaches embodied in the aforementioned patents, there is a continuing need for a lithographic printing plate that can be readily developed on a printing press and that produces a plate having durable image areas needed for good run length. Difficulty in the realization simultaneously of both "on-press developability" and "durability" is believed to originate from an apparent contradiction between photoresist removability ("developability") on the one hand and "durability" on the other: To make a photoresist more durable was to make the photoresist less developable.
The ramifications of the "durability/developability dilemma" are of little practical consequence to conventional plates which are not designed for on-press development. For such plates development and use will generally occur in separate stages. Accordingly, such plates will normally be treated first with a non-polar solvent developer or with an aqueous alkaline developer, and then, with aqueous acid fountain solutions during use on a printing press. The use of a developer whose solubility characteristics are dramatically different from the printing press use environment allows conventional resists to be formulated, for example, with wet-adhesion promoting functional groups or additives having different solubility characteristics depending on the relative alkalinity or acidity of its environment.
Certain problems arise in designing an "on-press" developable lithographic plate because the bath development environment associated with conventional plate development and the environmental conditions existing on a printing press for "on-press" plate development are different. If the photoresist is easily developed by the press fountain and ink solutions, swelling and the concomitant undercutting of the resist layer will occur due to the continued presence of such solutions during "use". Thus, in designing a practical and effective "on-press" developable lithographic printing plate, need arises for a mechanism whereby the solubility characteristics of the photoresist may be controlled for on-press development of the plate by fountain and ink solutions, without unacceptable compromise of the durability of the plate and the capacity of the plate to perform satisfactorily during continued use on the press.
Since it is impractical to modify the ink and fountain solutions of printing presses to accommodate "on press" developable plates, the present invention seeks to incorporate into photoresists a unique plasticizing system having components miscible with both the components of the host photoresist and typical fountain/ink solutions. In one aspect, the plasticizing systems are formulated to efficiently plasticize the photoresist to thereby enhance developability by facilitating fast diffusion of the fountain and ink solution into the photoresist. In a second aspect, the plasticized photoresist is also formulated to be dispersible or moderately soluble in typical fountain/ink combinations. Since the photoresist itself will typically have a low affinity toward the fountain and ink solutions, as the plasticizer is gradually leached out by the fountain/ink during plasticizer-enhanced development, the photoresist is concurrently converted gradually from a fountain/ink developable state to a fountain/ink resistant state. To this end, the present invention provides a plasticizing-permeation enhancing system generally comprising plasticizers with select solubility characteristics and, in certain embodiments, surfactants and, in still other embodiments, lithium salts. Properly composed, the durability of printing plates utilizing such plasticizing systems will not be compromised in exchange for enhanced developability.